1. Field of the invention
The present invention relates to an optical pumping unit comprising a first and a second pump source for providing two pump radiations, and a common coupling section for coupling a first, a second and a third radiation.
The present invention also relates to an optical amplifier comprising said optical pumping unit and an optical communication line and an optical communication system comprising said optical pumping unit or said optical amplifier.
The present invention also relates to a coupling section and a method for coupling two pump radiations and a signal radiation.
2. Technical Background
In the present description and claims, the expression
xe2x80x9cinsertion losses undergone by a pump radiationxe2x80x9d, referred to a pumping unit, is used to indicate the difference, expressed in dB, between the power of the radiation emitted by a pump source of the pumping unit and the power in output from the pumping unit;
xe2x80x9c100%xcex optical couplerxe2x80x9d is used to indicate an optical coupler comprising two optical paths coupled to one another and adapted to let pass substantially 100% of power of a radiation of wavelength xcex from one optical path to the other, and to substantially maintain 0% of power along the same optical path;
xe2x80x9c100%xcexx/0%xcexy WDM optical couplerxe2x80x9d is used to indicate an optical coupler comprising two optical paths coupled to one another and adapted to let pass from one optical path to the other substantially 100% of power of a radiation at wavelength xcexx, and substantially 0% of power of a radiation at wavelength xcexy by maintaining substantially 0% of power of the radiation at wavelength xcexx and substantially 100% of power of the radiation at wavelength xcexy along the same optical path;
xe2x80x9c50%xcexx/0%xcexy WDM optical couplerxe2x80x9d is used to indicate an optical coupler comprising two optical paths coupled to one another and adapted to let pass from one optical path to the other substantially 50% of power of a radiation at wavelength xcexx and substantially 0% of power of a radiation at wavelength xcexy by maintaining substantially 50% of power of the radiation at wavelength xcexx and substantially 100% of power of the radiation at wavelength xcexy along the same optical path;
xe2x80x9c50%xcexx/100%xcexy WDM optical couplerxe2x80x9d is used to indicate an optical coupler comprising two optical paths coupled to one another and adapted to let pass from one optical path to the other substantially 50% of power of a radiation at wavelength xcexx and substantially 100% of power of a radiation at wavelength xcexy by maintaining substantially the remaining 50% of power of the radiation at wavelength xcexx and substantially 0% of power of the radiation at wavelength xcexy along the same optical path;
xe2x80x9coptical transmission fibrexe2x80x9d is used to indicate an optical fibre used in an optical communication line or system for transmitting optical signals from a point to another placed at an appreciable distance.
In the above definitions, the expression xe2x80x9csubstantially 100%xe2x80x9d of power coupling is preferably used for indicating a power coupling at least equal to 90%; xe2x80x9csubstantially 0%xe2x80x9d of power coupling, is preferably used for indicating a power coupling that is less than or equal to 10%, and xe2x80x9csubstantially 50%xe2x80x9d of power coupling is preferably used for indicating a power coupling comprised between 45% and 55%.
A 100%xcexWDM, 100%xcexx/0%xcexy WDM, 50%xcexx/0%xcexy WDM, 50%xcexx/100%xcexy WDM optical coupler can be realised in micro-optics, fused fibre, integrated optics or through any other technique which allows the formation of waveguides at the optical frequencies.
An optical amplifier typically consists of an active means (for example, an optical fibre or a planar waveguide doped with a rare earth) and a pumping unit.
In turn, the pumping unit typically consists of a pump source (for example, a laser diode) adapted to provide a pump radiation at wavelength xcexp to the active means, and of a wavelength division multiplexing (or WDM) device for coupling the pump radiation at wavelength xcexp with a signal radiation to be amplified at wavelength xcexs.
Typically, the WDM device is a WDM optical coupler of the 100%xcexp/0%xcexs or 100%xcexs/0%xcexp type, with two inputs and two outputs, and it is adapted to couplexe2x80x94into one of the two outputsxe2x80x94substantially all the power of the pump radiations at wavelength xcexp and of the signal radiations at wavelength xcexs at its two inputs.
With the advent of WDM optical communication systems, there is the need of increasing the pump power of optical amplifiers, so as to effectively amplify a WDM optical signal.
A WDM optical signal is a signal comprising a plurality of N optical signals independent of one another, each at a predetermined central wavelength xcex1, xcex2 . . . xcexN different from that of the other signals. The signals can also be both digital and analogue, and they have a certain spectral width around the value of the central wavelength.
Typically, in a WDM system, the various optical signals are generated by a plurality of optical sources, multiplexed so as to form a WDM signal, transmitted along the same optical transmission line (for example an optical fibre line) and then demodulated so as to be each received by a receiver.
Optical amplifiers used in transmission, reception and/or along a transmission line of a WDM optical system need high pump powers to efficiently amplify the plurality of optical signals forming the WDM optical signal.
For the purpose of meeting said requirement, the use of a pumping unit with two pump sources has been proposed.
More in particular, it has been proposed to combine two pump radiations provided by two pump sources in a single total pump radiation, and to combine said total pump radiation with the signal radiation.
For example, the use of a wavelength combiner or of a polarisation combinerxe2x80x94upstream of a WDM device used for coupling the total pump radiation with the signal radiationxe2x80x94has been proposed to combine the two pump radiations.
In the first case (FIG. 1), the pumping unit 10 comprises two laser diodes 11, 12, two optical fibre gratings 15, 17 respectively connected to the two lasers 11 and 12, a wavelength combiner 14 and a fused fibre 100%xcexp/0%xcexs WDM optical coupler 16. According to this solution, the pump radiations emitted by the two laser diodes 11, 12 have slightly different wavelengths (typically, by some nm) and the two optical fibre gratings 15, 17 are adapted to stabilise said wavelengths.
Since the wavelengths of the two pump radiations are very close to one another (by some nm), the wavelength combiner 14 is typically realised in micro-optics. In fact, a fused fibre 100%xcexp/0%xcexs WDM optical coupler of the type used for coupling the total pump radiation and the signal radiation (which typically have different wavelengths from one another, in the range of dozens or hundreds nm), is not adapted to combine wavelengths that are very close to one another (in the range of nm).
Considering Bragg gratings currently available on the market by JDS, E-TeK, Innovative Fibers, Sumitomo, Bragg Photonics, 3M, Optical Technologies Italia (having insertion losses that are more than or equal to about 0.2 dB) and micro-optics wavelength combiners and fused fibre 100%xcexp/0%xcexs WDM optical couplers currently available on the market by JDS, E-TeK, Oplink and Gould (having insertion losses that are respectively higher than or equal to about 0.6 dB and 0.3 dB), the Applicant has noted that in the pumping unit of FIG. 1 each pump radiation undergoes insertion losses higher than about 1.1 dB (that is, higher than about 23%).
Moreover, as the pumping unit of FIG. 1 consists of the micro-optics wavelength combiner 14, the two optical fibre gratings 15, 17 and the fused fibre 100%xcexp/0%xcexs WDM optical coupler 16, it is realised using different technologies. This makes the pumping unit less reliable and more expensive than a unit that is entirely realised with the same technology (for example, all in fibre or all in micro-optics).
In the second case of use of a polarisation combiner (FIG. 2), the pumping unit 10 comprises two laser diodes 11, 12 with polarisation-holding pigtail 11a, 12a, a polarisation combiner 13 and a fused fibre 100%xcexp/0%xcexs WDM optical coupler 16. According to this solution, the pump radiations emitted by the two laser diodes 11, 12 have the same wavelength, and the two pigtails 11a, 12a make said pump radiation have orthogonal polarisation states.
The Applicant checked that in this second pumping unit of FIG. 2, insertion losses undergone by the pump radiations are comparable to those undergone by the pump radiations in the pumping unit of FIG. 1.
Moreover, since the pumping unit of FIG. 2 requires the use of polarisation-maintaining components, it is difficult and expensive to be realised.
M. Ohashi et al. (xe2x80x9cNovel pump-LD with self wavelength-tuning functionxe2x80x9d, ECOC 2000) describe a pumping module comprising four laser diodes with emission at the wavelengths of 980, 982, 981, 983 nm, a wavelength combiner and a wide band optical fibre Bragg grating (FBG). In turn, the wavelength combiner consists of three cascaded fused-tapered Mach-Zehnder interferometers.
The pump radiations combined with the described module are intended to be coupledxe2x80x94through a distinct WDM devicexe2x80x94with the radiation of the signal to be amplified, to be sent along an active optical fibre of an optical amplifier.
Nevertheless, the Applicant has noted that also in this case, the insertion losses undergone by the pump radiations are equally high (about 1.1 dB).
The Applicant has noted that the above proposed solutions all have a coupling section of the pump radiation which is clearly distinct from the coupling section of the total pump radiation with the signal radiation. This makes it necessary to use a certain number of optical components in cascadexe2x80x94such as, for example, polarisation combiners, 100%xcexx/0%xcexy WDM optical couplers, micro-optics wavelength combiners, Mach-Zehnder interferometersxe2x80x94which introduce undesired insertion losses on the pump radiations.
Thus, the Applicant faced the technical problem of reducing the insertion losses undergone by the pump radiations of an optical pumping unit having at least two pump sources.
The Applicant has found that, by using a common coupling section for mixing at least two pumping radiations and the signal radiation, the optical power losses undergone by the pump radiations significantly reduce.
Thus, in a first aspect thereof, the present invention relates to an optical pumping unit comprising
a first pump source adapted to emit a first pump radiation at wavelength xcexp1;
a second pump source adapted to emit a second pump radiation at wavelength xcexp2, with wavelength xcexp2 different from wavelength xcexp1; and
a common coupling section comprising
a first and a second port connected to the first and second pump source for respectively receiving the first and the second pump radiation;
a third port for a signal radiation at wavelength xcexs;
a fourth port,
wherein said coupling section is adapted to combine, in the fourth port, the signal radiation and the first and second pump radiation by means of a reversal of the direction of propagation of the first pump radiation from the first port to the fourth port.
Since the optical pumping unit of the invention uses a common coupling section with all of the above features for coupling pump and signal radiations, it eliminates the need of using distinct coupling sections for combining the pump radiations in a total pump radiation and the total pump radiation with the signal radiation. Thus, it allows using a limited number of optical components in cascade, thus reducing the insertion losses undergone by the pump radiations.
Moreover, by using a limited number of optical components in cascade, the optical pumping unit of the invention is more compact and less expensive to be realised than the above-mentioned conventional pumping units.
In fact, a limited number of optical components allows simplifying the step of assembly of the pumping unit (thus reducing, for example, the number of junctions to be made between the components) and limiting production times and costs.
Typically, the wavelength xcexs of the signal radiation is higher than wavelengths xcexp1 and xcexp2 of the pump radiations.
Advantageously, the difference (xcexsxe2x88x92xcexpmax) between wavelength xcexs and the highest xcexpmax between wavelengths xcexp1 and xcexp2 is equal to at least 30 nm. In a preferred embodiment, it is equal to at least 530 nm.
Advantageously, the difference between wavelengths xcexp1 and xcexp2 is less than or equal to, 30 nm. Preferably, it is less than or equal to, 20 nm. More preferably, it is less than or equal to, 10 nm.
Typically, in the case of application of the pumping unit for pumping an erbium-doped optical amplifier, wavelengths xcexp1 and xcexp2 are selected within an interval of wavelengths comprised between about 975 and 985 nm and/or 1470 and 1490 nm whereas wavelength xcexs is selected within an interval of wavelengths comprised between about 1520 and 1630 nm.
In turn, in the case of application of the pumping unit for mixing three signal radiations in the treatment of WDM signals, wavelengths xcexp1 and xcexp2 are, for example, selected at about 1530 nm and respectively, 1550 nm whereas wavelength xcexs is selected at about 980nm.
Typically, the coupling section has a first and a second side, the one opposed to the other. Advantageously, the first and the fourth port of the coupling section are located at the first side, while the second and the third port are at the second side, in positions respectively corresponding to the first and the fourth port.
Preferably, the coupling section comprises
a first optical path which connects the first and the second port; and
a second optical path, in communication with the first optical path, which connects the third and the fourth port,
and it is adapted to send to the fourth port the first pump radiation, which propagates along the first optical path from the first port to the second port, making it pass from the first optical path to the second optical path and reflecting it back towards the fourth port.
Advantageously, the coupling section is also adapted to send to the fourth port the second pump radiation, which propagates along the first optical path from the second port towards the first port, making it pass from the first optical path to the second optical path.
Moreover, the coupling section is also preferably adapted to let the signal radiation propagate along the second optical path.
According to an embodiment, the signal radiation propagates along the second optical path from the third port to the fourth port.
According to an alternative embodiment, the signal radiation propagates along the second optical path from the fourth port towards the third port.
Advantageously, the coupling section comprises an optical reflection element adapted to reflect the first pump radiation at wavelength xcexp1 towards the fourth port, and to let the second pump radiation at wavelength xcexp2 and the signal radiation at wavelength xcexs pass.
Preferably, said optical reflection element is a Bragg grating. As an alternative, said optical reflection element is a thin-film optical filter, such as a Fabry-Perot interferometer.
Preferably, the coupling section is of the interferometric type.
Advantageously, the first optical path comprises a waveguide.
Advantageously, the second optical path comprises a waveguide.
Preferably, said waveguide is an optical fibre. According to an alternative, it is a planar waveguide realised in integrated optics.
Preferably, the first and the second optical path are coupled along a coupling area.
More preferably, the coupling area is such as to let substantially all the power of the signal radiation at wavelength xcexs propagate along the second optical path, and to let substantially all the power of the first pump radiation at wavelength xcexp1 and substantially all the power of the second pump radiation at wavelength xcexp2 pass from the first optical path to the second optical path.
Advantageously, the first and the second optical path form a WDM optical coupler of the 100%xcexp1, xcexp2/0%xcexs type, comprising two waveguides coupled with one another in said coupling area.
Preferably, the 100%xcexp1, xcexp2/0%xcexs WDM optical coupler is a fused fibre optical coupler. According to an alternative, it is realised in integrated optics (for example, in planar waveguide).
Preferably, the optical reflection element is positioned in the coupling area of the first and the second optical path.
More preferably, the optical reflection element is positioned in a point of the coupling area at which about 50% of power of the first pump radiation passes from the first optical path to the second optical path.
According to an embodiment, the first and the second optical path are also coupled along a second coupling area.
In the optical pumping unit according to this embodiment, the first and the second optical path advantageously comprise an input coupler, an output coupler, an upper arm and a lower arm. Moreover, the input coupler has four ports of which two are the second and the third port of the coupling section, and two are in communication with the upper arm and the lower arm, while the output coupler has four ports of which two are the first and the fourth port of the coupling section, and two are in communication with the upper arm and the lower arm.
In the optical pumping unit according to this embodiment, the coupling section preferably comprises also a second optical reflection element adapted to reflect the first pump radiation at wavelength xcexp1 towards the fourth port, and to let the second pump radiation at wavelength xcexp2 and the signal radiation at wavelength xcexs pass, the first optical reflection element being arranged in said upper arm and the second optical reflection element being arranged in said lower arm.
The input coupler and the output coupler preferably are two WDM optical couplers of the 50%xcexp1, xcexp2/0%xcexs type, each comprising two waveguides coupled with one another in said first and said second coupling area.
The input and output optical couplers and the two upper and lower arms are preferably realised in optical fibre. According to an alternative, they are realised in integrated optics (for example, in planar waveguide).
In a second aspect thereof, the invention also relates to an optical amplifier for amplifying a signal radiation at wavelength xcexs comprising a dielectric guiding active means and a pumping unit of the type described above with reference to the first aspect of the invention wherein the fourth port of the coupling section is in communication with the active means.
Advantageously, the active means is an optical waveguide doped with at least one rare earth. Typically, said at least one rare earth is erbium.
Typically, the doped optical waveguide is an optical fibre or a planar waveguide realised in integrated optics.
In a third aspect thereof, the invention also relates to an optical communication line comprising a transmission optical fibre length and a pumping unit of the type described above with reference to the first aspect of the invention wherein the fourth port of the coupling section is in communication with said transmission optical fibre length.
In a fourth aspect thereof, the invention also relates to an optical communication line comprising a transmission optical fibre length and an optical amplifier, of the type described above with reference to the second aspect of the invention, in communication with said transmission optical fibre length.
In a fifth aspect thereof, the present invention also relates to an optical communication system comprising
a transmitting station adapted to provide a signal radiation having wavelength xcexs;
an optical transmission line, optically connected to said transmitting station, for transmitting said signal radiation;
a receiving station, optically connected to said optical transmission line, for receiving said signal radiation;
at least one pumping unit, of the type described above with reference to the first aspect of the invention, in communication with said optical transmission line.
In a sixth aspect thereof, the present invention also relates to an optical communication system comprising
a transmitting station adapted to provide a signal radiation having wavelength xcexs;
an optical transmission line, optically connected to said transmitting station, for transmitting said signal radiation;
a receiving station, optically connected to said optical transmission line, for receiving said signal radiation;
at least one optical amplifier, of the type described above with reference to the second aspect of the invention, in communication with said optical transmission line.
Advantageously, said transmitting station is adapted to provide a WDM optical signal comprising a plurality of N signals having wavelengths xcex1, xcex2 . . . xcexN.
In this case, said receiving station is advantageously adapted to receive and demultiplex said WDM optical signal.
In a seventh aspect thereof, the present invention also relates to an optical coupling section for coupling a signal radiation at wavelength xcexs, a first pump radiation at wavelength xcexp1 and a second pump radiation at wavelength xcexp2, comprising
a first and a second port for receiving respectively the first and the second pump radiation;
a third port for the signal radiation; and
a fourth port,
and being adapted to combine the signal radiation and the first and second pump radiation in the fourth port through a reversal of the direction of propagation of the first pump radiation from the first port to the fourth port.
As regards the features of the coupling section and of the pump and signal radiations, reference shall be made to what described above with reference to the pumping unit according to the first aspect of the invention.
In an eighth aspect thereof, the present invention also relates to an optical coupling section, for coupling a first radiation at wavelength xcexp1, a second radiation at wavelength xcexp2 and a third radiation at wavelength xcexs, comprising
a first and a second port for respectively receiving the first and the second radiation;
a third port for the third radiation; and
a fourth port,
and being adapted to combine the first, the second and the third radiation in the fourth port through a reversal of the direction of propagation of the first radiation from the first port to the fourth port.
As regards the features of the coupling section of the first, second and third radiation, reference shall be made to what described above with reference to the pumping unit according to the first aspect of the invention, and with reference to the first and second pump radiation and to the signal radiation.
In a further aspect thereof, the present invention also relates to a method for coupling a first radiation at wavelength xcexp1, a second radiation at wavelength xcexp2 and a third radiation at wavelength xcexs through a common coupling section having a first and a second side that are opposed to one another, the first side comprising a first and a fourth port and the second side comprising a second and a third port, said method comprising the steps of
a) propagating the second radiation from the second port to the first port;
b) deviating the path of the second radiation so as to send it to the fourth port;
c) sending the third signal radiation from the third port to the fourth port, or vice versa, from the fourth port to the third port;
d) propagating the first radiation from the first port to the second port; and
e) reversing the direction of propagation of the first radiation to send it to the fourth port.
Advantageously, the common coupling section also comprises a first optical path connecting the first and the second port, and a second optical path, in communication with the first optical path, connecting the third and the fourth port.
Preferably, step a) is carried out by sending the second radiation along the first optical path from the second port to the first port.
Moreover, step b) is preferably carried out by making the second radiation pass from the first optical path to the second optical path.
Advantageously, step c) is carried out by letting the third radiation propagate along the second optical path.
Preferably, step d) is carried out by sending the first radiation along the first optical path from the first port to the second port.
Moreover, step e) is preferably carried out by making the first radiation pass from the first optical path to the second optical path and back-reflecting it towards the fourth port.
Advantageously, the passage from the first to the second optical path of steps b) and e) occurs by interferometric effect.
As regards the features of the coupling section and of the first, second and third radiation, reference shall be made to what described above with reference to the pumping unit according to the first aspect of the invention and to the first and second pump radiation and to the signal radiation.